Modern air-purifying respirators targeted to filter a range of threat agents, such as chemical, biological, radiological and nuclear (CBRN) respirators, rely on filter elements that have limited service lives. While these respirators are intended to be worn for protection against threat agent airborne concentrations that are dangerous to life and health, presently, the respirators provide no direct indication of their remaining gas-life capacity. Current military doctrine for determining how often to exchange CBRN mask filters is therefore based on evaluating factors calculated to indirectly indicate the remaining gas life, such as the physical condition of the filter, the type and extent of threat agent exposure, climatic conditions, and other criteria that are known to affect service life. The uncertain and subjective nature of these factors and the consequences of miscalculation have lead to widespread premature disposal of filters. For example, according to military doctrine, during wartime operations, respirator filters in masks that have been worn in areas previously exposed to a chemical attack are to be disposed of after 30 days. In actual practice, however, the respirator filters are often exchanged in a combat environment every 30 days whether or not there has been a confirmed chemical attack. These change-out practices, of course, are deliberately conservative but they impose substantial additional costs and logistic burdens on military and civilian authorities responsible for maintaining an adequate supply of replacement filters. Even conservative filter change-out practices provide no absolute assurance that a respirator filter is still effective.
Military and emergency responder communities including security and law enforcement personnel, tactical response units, health care workers, and a growing number of other users require a more reliable and objective means to determine when to replace a CBRN filter. Embodiments according to the present invention address these concerns, at least in part.